1. Technical Field
The present invention relates to an active matrix type liquid crystal display device, more particularly to a so-called vertical alignment mode liquid crystal display device in which an electric field in a vertical direction is applied to a liquid crystal layer having liquid crystal molecules possessing a negative dielectric anisotropy, the molecules being oriented in the vertical direction, and the liquid crystal molecules are allowed to orient in a horizontal direction, thus controlling a light transmission/cutting-off.
2. Prior Art
A widely spread conventional display device using liquid crystal materials employs a liquid crystal layer of a twisted nematic liquid crystal. In this device, an electric field is applied in a direction perpendicular to the substrate. In this system, polarizing plates are arranged on the two sides of the liquid crystal layer in such a way that their polarizing axes are perpendicular. Liquid crystal molecules are oriented in a vertical direction when an electric field is applied and a black image display is acquired. However, upon the application of the electric field, a light transmitting obliquely through the liquid crystal layer in which the liquid crystal molecules are oriented in the vertical direction undergoes double refraction due to the liquid crystal molecules, and its polarizing direction is rotated. Therefore, there has been a problem that a perfectly black displaying can not be acquired when the display device is viewed obliquely, quality contrast of the display is lowered, and viewing angles at which a satisfactory image can be observed is narrow.
In order to solve such problem, a liquid crystal display device which utilizes a so-called in-plane switching (IPS) mode in which the direction of an electric field applied to a liquid crystal is parallel to the substrate has been recently proposed. In case of the IPS mode, since the liquid crystal molecules rotate primarily in a plane parallel to the substrate, the degree of a double refraction differs little when viewed obliquely in a state when the electric field is applied and when it is not applied. Consequently, it is known that view angle is widened. However, in the IPS mode liquid crystal display device, since a comb-shaped opaque electrode is, for example, provided on one of the substrates existing on both sides of the liquid crystal layer, this liquid crystal display device involves a problem that an aperture ratio decreases. The IPS mode liquid crystal display device having a low aperture ratio is disadvantageous for portable equipment such as a notebook type personal computer in which low power consumption is required, and so it is generally used at present as a standalone monitor equipment.
As a method to obtain a liquid crystal display device having a wide viewing angle and a high aperture ratio and thereby to solve the above described problems, a vertical alignment mode liquid crystal display device has been proposed. In this liquid crystal display device, a liquid crystal layer formed of liquid crystal molecules possessing a negative dielectric anisotropy is provided between two parallel plate type electrodes. The liquid crystal molecules are oriented perpendicularly to the electrodes when no voltage is applied to the electrodes. Moreover, the upper and lower polarizing plates are so arranged that their polarizing axes are perpendicular to each other. When the voltage is not applied, an incident light straightly travels through the liquid crystal layer, so that the light passes through one polarizing plate and is linearly polarized. This light is perfectly shielded by the other polarizing plate, and thus the display becomes black. When a voltage is applied, the liquid crystal molecules possessing a negative dielectric anisotropy are oriented in a direction perpendicular to the direction of the electric field, that is, the liquid crystal molecules are oriented in a direction parallel to the electrodes. In this case, since the incident light is rotated by the liquid crystal layer exhibiting a retardation, the linearly polarized light polarized by the polarizing plate travels through the other polarizing plate and the display becomes white.
In the vertical alignment mode liquid crystal display device, although a contrast when viewed from the front is extremely good, a contrast when viewed from upper and lower directions as well as left and right directions becomes worse as compared to that in the case of IPS mode liquid crystal display device. For this reason, an optical compensation sheet is employed, however, detailed studies have not been made as to the direction and magnitude of a retardation in an in-plane direction of the sheet, and the magnitude of a retardation in a direction perpendicular to the sheet. In SID""98, Chen et. al. disclose that a perfect compensation is possible by using two biaxial sheets (SID""98 DIGEST page 315). This aims at achieving a rotation of a polarizing axis by a retardation of about 50 nm in-plane direction. Although it was experimentarily confirmed that the optical compensation is possible according to this method, the reason of this was not perfectly made clear. Moreover, two optically biaxial anisotropic sheets provided on upper and lower sides of the liquid crystal display device are required.
The vertical alignment mode liquid crystal display device inherently has a drawback concerning a visual angle property. In the vertical alignment mode liquid crystal display device, by using liquid crystal molecules homogeneously oriented in a vertical direction and two polarizing plates disposed so that their transmitting axes intersect at a right angle, a black displaying is performed. When viewing obliquely as compared to the direction of the transmitting axis of the polarizing plates, the polarizing plates have a positional relationship in which the two transmitting axes look to intersect at a right angle and the homogeneous orientation liquid crystal layer exhibits a double refraction which is less as compared to in the twisted mode liquid crystal layer. Therefore, a completely black displaying can be achieved. On the contrary, when viewed obliquely from a direction that makes an angle of 45 degrees with the transmitting axis of the polarizing plate, as is apparent from the fact that the two polarizing plates have a positional relationship in which an angle formed by their transmitting axes looks to be shifted from 90 degrees, the transmitting light causes a double refraction and the light leaks. Therefore, a satisfactory black display can not be achieved, and the contrast is degraded. The vertical alignment mode liquid crystal device exhibits a small difference in retardation values between when viewed from a front direction and when viewed from an oblique direction. Therefore, when viewed from the 45 degrees direction, the degradation of the contrast in the vertical alignment mode is more noticeable than in the IPS mode liquid crystal display device.
FIG. 1 shows computation results for contrast lines in the conventional vertical alignment mode liquid crystal display device. The black portion shows a region where the contrast is 50 or more. It is proved that a decline in the contrast at an angle of 45 degrees with respect to the polarizing axis of the polarizing plate occurs in four directions, i.e., azimuth angles of 0 degree, 90 degrees, 180 degrees and 270 degrees. As a result of the decline in the contrast in the four directions, an inversion of luminance occurs in the region ranging from black to intermediate tone. Such decline in the contrast in the four directions is the drawback of the vertical alignment mode liquid crystal display device which exhibits an excellent visual angle property. The object of the present invention is to provide a vertical alignment mode liquid crystal display device which is capable of perfectly achieving a contrast compensation within the region ranging all over the azimuth angles of 360 degrees and at the region ranging from the polar angle of 0 to 80 degrees.
The present invention relates to a liquid crystal display device which comprises a first polarizing plate having a transmitting axis in a specified direction; an optically biaxial anisotropic sheet; a liquid crystal layer formed of vertically aligned nematic liquid crystal molecules possessing a negative dielectric anisotropy, in this order; and a second polarizing plate having transmitting axis perpendicular to the foregoing specified direction; an optically uniaxial anisotropic sheet selectively being provided either between the foregoing liquid crystal layer and the foregoing optically biaxial anisotropic sheet or between the foregoing liquid crystal layer and the foregoing second polarizing plate.
The present invention relates further to a liquid crystal display device in which a retardation of the optically uniaxial anisotropic sheet ranges from 75 to 100% of that of the foregoing liquid crystal layer.
Still further, the present invention relates to a liquid crystal display device in which an in-plane retardation of the foregoing optically biaxial anisotropic sheet is from 190 to 390 nm.
Still even further, the present invention relates to a liquid crystal display device in which a retardation in a thickness direction of the foregoing optically biaxial anisotropic sheet is from 0.28 to 0.67.
Yet even further, the present invention relates to a liquid crystal display device in which an in-plane retardation of the foregoing optically biaxial anisotropic sheet is from 205 to 315 nm, and a retardation in a thickness direction of the foregoing optically biaxial anisotropic sheet is from 0.3 to 0.65.
Still further, the present invention relates to a liquid crystal display device in which an in-line retardation axis of the foregoing optically biaxial anisotropic sheet is approximately parallel to the transmitting axis of either the foregoing first or second polarizing plate.
The present invention even further relates to a liquid crystal display device in which the foregoing optically uniaxial anisotropic sheet is provided integrally with the foregoing second polarizing plate.
And further, the present invention relates to a liquid crystal display device in which the foregoing optically uniaxial anisotropic sheet is tri-acetyl cellulose.